How do I change the direction of an object set to "Bake Sound to F-Curves"?
Idle Chit-chat

I assume that by "in a downward motion" you mean that you want its jittering effectively reflected over the x axis. There is a way to do this.

Solution

Go to the Graph Editor, select the baked-sound f-curve, key N to open the Graph Editor's Properties panel, and open the Modifiers drop down menu (it should be at the bottom of the stack). Add an Envelope modifier and change the settings to this:



You will need to click Add Point before you can change the bottom-most settings (1, 0, and -1.8). This should make the cube move in a downward motion.